Tracking recent rigify changes?
So, I ended up modifying limb_rigs.py @ line 1014 with the following prints

print("Dir Result", dir(self.properties))
print("ctrl_bones",self.properties.ctrl_bones)
print("extra_ctrls",self.properties.extra_ctrls)
print("fk_bones",self.properties.fk_bones)
print("ik_bones",self.properties.ik_bones)
print("pole_prop",self.properties.pole_prop)
print("prop_bone",self.properties.prop_bone)
print("use_pole",self.properties.use_pole)


this outputs the results of a signature enough to pinpoint any new bones and variables that need passing.

It seems to me that the current implementation of the function suffers from a bug when variables are left empty as they should be.

fk_bones is always empty. but a value is required for calling the function because of it's type.
As a result you need a call of this sort

bpy.ops.pose.rigify_limb_toggle_pole_wlnrc42c2a6335e5(
prop_bone="upper_arm_parent.L", 
pole_prop="pole_vector", 
fk_bones='[""]',
ik_bones='["upper_arm_ik.L", "MCH-forearm_ik.L", "MCH-upper_arm_ik_target.L"]', 
ctrl_bones='["upper_arm_ik.L", "hand_ik.L", "upper_arm_ik_target.L"]', 
extra_ctrls='[""]', 
use_pole=True)


Glad I sorted it.
Still if someone from the Rigify team eventually spots this I'd like you to consider NOT changing function signatures without documenting and publishing your changes...